The present invention relates to a cobalt base alloy (referred to as Co base alloy hereinafter) and, more particularly, to a Co base cast alloy having a superior high-temperature strength and a high-temperature ductility.
Co base alloys have been used as the materials for mechanical parts which are subjected to drastic and repeated heating and cooling. A typical example of such mechanical parts is the first stage nozzle of a gas turbine. The first stage nozzle of the gas turbine is usually designed to withstand a long use of 20 to 30 thousand hours or longer at a high temperature of 800.degree. to 1000.degree. C. The condition of use of the first stage nozzle of gas turbine is very severe.
A Co base superheat resisting cast alloy is produced by a precision casting. The development of Co base heat-resisting superalloy has been made hitherto aiming mainly at achieving a superior high-temperature strength, particularly a high creep rupture strength. As a result, the conventional Co base heat-resisting superalloys disadvantageously exhibit an inferior high-temperature ductility, although the high-temperature strength have been improved considerably.
On the other hand, various investigations and studies show that the cracking of the first stage nozzle at gas turbine which is often observed during use of the gas turbine is attributable to a repetition of thermal stress, i.e. to a thermal fatigue, rather than to a lack of the high-temperature strength. The thermal fatigue is affected by high-temperature ductility, as well as the high-temperature strength, of the material. The conventional Co base cast alloys for nozzles which have been developed mainly aiming at a high-temperature strength possess sufficiently high creep rupture strengths. In fact, these conventional Co base cast alloy exhibit ductility against creep rupture of a level exceeding that required practically up to a temperature below 900.degree. C. As the temperature is increased up to, for example, 982.degree. C., the ductility is drastically lowered. For example, the elongation ratio in a long-term creep rupture test for 1,000 hours or longer is as small as several percents. In consequence, cracks are often caused in the nozzle of the gas turbine when the nozzle is used at a temperature exceeding 900.degree. C. For this reason, it is desirable that the nozzle material is superior in both of the high-temperature strength and high-temperature ductility and that the high-temperature ductility is not lowered even at temperatures above 900.degree. C. A superior high-temperature ductility offers another advantage of easy repair of the nozzle by welding, even if a crack is generated in the nozzle.
The conventional Co base alloy exhibits a high-temperature ductility which is high at the temperature range below 900.degree. C. but decreases drastically as the temperature is increased beyond 900.degree. C. This is attributable to the following reason.
Namely, conventional Co base alloy is made to have a large chromium (Cr) content in order to compensate for the low anti-oxidation characteristic of the Co. Therefore, film-like non-metallic inclusions, which are possibly oxides, are formed in the grain boundaries during the casting to hinder the free deformation of the grains. The reduction of ductility can be attributed to this difficulty in deformation of the grains.
More specifically, at the temperature below 900.degree. C., the ductility of the matrix is large enough to overcome the influence of the non-metallic inclusions in the grain boundaries, because the amount of precipitation is not so large at such a comparatively low temperature. The alloy, therefore, exhibits a sufficiently high ductility at such a low temperature. However, at the high-temperature such as 982.degree. C., precipitation of carbides in the matrix is promoted to make the deformation of the same difficult. In consequence, the influence of the non-metallic inclusions in the grain boundaries become more strong. Namely, at the temperature range above 982.degree. C., the deformation at the grain boundaries is hindered by the non-metallic inclusions so that the ductility of the alloy is considerably lowered. The non-metallic inclusions are considered to be oxides, although the detail of structure thereof has not been clarified yet. Also, a large Cr content promotes the precipitation of nitrides at the high-temperature exceeding 982.degree. C., which also causes the reduction of ductility of the alloy.
On the other hand, in the Co base cast alloy having a high Cr concentration, a grain boundary oxidation is caused in the atmosphere to lower the ductility of the alloy. Generally, a high-strength Co base cast alloy contains solid solution hardeners such as tungsten (W), molybdenum (Mo) or the like and is reinforced by a formation of carbides. The carbides are formed in the network-like pattern in the grain boundaries.
The carbides are liable to be selectively oxidized at high-temperature, while the diffusion of oxygen is made rapidly in the grain boundaries. As a result, the oxidation of the network carbides in the grain boundaries is accelerated. This acceleration of the oxidation in the grain boundaries in turn causes a concentration of stress, resulting in a reduction of the strength, as well as ductility.
In order to increase the high-temperature strength of the Co base cast alloy, it has been proposed to add a large amount of strong carbide formers such as titanium (Ti), zirconium (Zr), niobium (Nb), tantalum (Ta) and the like. This type of Co base cast alloy is disclosed, for example, in the specification of U.S. Pat. No. 3,432,294.
Also, specifications of U.S. Pat. Nos. 3,582,320 and 4,080,202 discloses Co base cast alloy in which traces of these elements are contained.
These Co base cast alloys, however, are still insufficient to provide the required high-temperature strength and high-temperature ductility.
(1) As disclosed in the specification of U.S. Pat. No. 4,080,202, the present inventors discovered that a heat treated cobalt base cast alloy containing Ti and Nb which are strong MC-type carbide formers exhibits high strength and high ductility. However, it was difficult to sufficiently smooth the casting surface of this alloy in as-cast state, and it was discovered that when a very small amount of zirconium is added to this alloy the casting surface becomes good.
Also it was found that the casting surface degrading effect of titanium is larger than that of niobium.
Recently, there is an increasing demand for development of gas turbines having high efficiency. The high efficiency of the gas turbine is achievable by an extremely high gas temperature which well reaches 1,300 to 1,600.degree. C. at the turbine inlet. This in turn gives rise to the demand for a higher strength against the thermal stress of the nozzle material. For instance, the nozzle material is required to have a high rupture strength of 4.3 Kg/mm.sup.2 or higher in 1,000 hours creep rupture strength test at 982.degree. C., as well as a high reduction of area of at least 20% in 100 hours rupture test at the same temperature.